What is a Polar Covalent Bond?

A covalent bond is formed when two atoms share electrons between them. The covalent bond is likely polar when the electrons are unequally shared. The skewness arises due to the difference in the electronegativities of the two atoms. Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. It is measured using a scale known as the Pauling scale and ranges from 0.7 to 4. The following table explains the type of chemical bond formed due to this electronegativity difference ^[1-10].

Type of Chemical Bond	Electronegativity Difference
Nonpolar covalent	0
Slightly polar covalent	0.1 to 0.4
Polar covalent	0.5 to 2
Ionic	> 2

The more significant the difference in electronegativity, the greater the polarity and the bond strength. Hence, polar covalent bonds have an intermediate bond strength.

Properties of Polar Covalent Bonds

Here are some facts about polar covalent bonds.

• It takes place between two nonmetals or between a nonmetal and a metalloid
• Unequal sharing of electrons in a compound
• The electronegativity difference between atoms is between 0.1 and 2
• One end of the bond is positively charged, and the other end is negatively charged
• Polar compounds dissolve in a polar solvent

Examples of Polar Covalent Bonds

Here are some examples of a polar covalent bond with the electronegativity difference between the two bonding atoms ^[1-6]:

Compound Name	Molecular formula	Bond	Electronegativity difference [1]
Water	H2O	O-H	1.4
Hydrogen fluoride	HF	H-F	1.9
Hydrogen chloride	HCl	H-Cl	0.9
Hydrogen bromide	HBr	H-Br	0.7
Ammonia	NH3	N-H	0.9
Sulfur dioxide	SO2	S=O	1
Hydrogen sulfide	H2S	H-S	0.4
Methanol	CH3OH	O-H	1.4

An explanation of the polar covalent bond in some compounds is given below.

1. Water (H₂O)

Water is a polar solvent. A polar covalent bond is created when the oxygen (O) atom, more electronegative than hydrogen, pulls the shared electrons towards itself. As a result, the oxygen atom has a partial negative charge associated with it. On the other hand, the hydrogens (H) are essentially protons and have a partial positive charge.

2. Hydrogen Chloride (HCl)

Hydrogen chloride is a polar covalent compound as the chlorine (Cl) atom is more electronegative than the hydrogen (H) atom. So, chlorine carries a partial negative charge, while hydrogen carries a partial positive charge.

3. Ammonia (NH₃)

Ammonia is a polar molecule. The central atom, nitrogen (N), is bonded to three hydrogen (H) atoms. Since N is more electronegative than H, the electrons will tend to hang around the N atom.

4. Hydrogen Bromide (HBr)

The difference in electronegativity between hydrogen (H) and bromine (Br) is not too high. Therefore, the HBr gas has a covalent bond that is polarized. The hydrogen atom has a slight positive charge, and the bromine has a slight negative charge. As a result, the electron pair is attracted to the bromine.

Difference Between Polar and Nonpolar Covalent Bonds ^[3]

The following table compares and contrasts polar and nonpolar covalent bonds.

Polar Covalent Bond vs. Nonpolar Covalent Bond
	Polar	Nonpolar
Types of atoms	Between two nonmetals with different electronegativities	Between two nonmetals of same electronegativities
Electronegativity difference	0.1 – 2	0
Electron distribution	Asymmetrical. Unequal sharing.	Symmetrical. Equal sharing.
Displacement of shared electrons	Towards the more electronegative atom, making that part negative	No displacement. Electrically neutral.
Dipole moment	Non-zero	Zero
Presence of other types of bonds in compounds	Hydrogen bond	Van der Waals bond
Affinity towards water	Hydrophilic	Hydrophobic
Physical properties of the compounds	High melting and boiling points	Low melting and boiling points
Volatility of liquids	Low volatility	High volatility
Solubility of compounds	Soluble in polar solvents	Soluble in nonpolar solvents
Examples	Water (H2O), ammonia (NH3), and hydrogen chloride (HCl)	Hydrogen (H2), oxygen (O2), and nitrogen (N2)